Turbogenerator power control system

ABSTRACT

A power control system for a turbogenerator which provides electrical power to one or more pump-jack oil wells. When the induction motor of a pump-jack oil well is powered by three-phase utility power, the speed of the pump-jack shaft varies only slightly over the pumping cycle but the utility power requirements can vary by four times the average pumping power. This power variation makes it impractical to power a pump-jack oil well with a stand-alone turbogenerator controlled by a conventional power control system. This power control system comprises a turbogenerator inverter, a load inverter, and a central processing unit which controls the frequency and voltage/current of each inverter. Throughout the oil well&#39;s pumping cycle, the central processing unit increases or decreases the frequency of the load inverter in order to axially accelerate and decelerate the masses of the down hole steel pump rods and oil, and to rotationally accelerate and decelerate the masses of the motor rotors and counter balance weights. This allows kinetic energy to be alternately stored in and extracted from the moving masses of the oil well and allows the oil pumping power to be precisely controlled throughout the pumping cycle, resulting in a constant turbogenerator power requirement.

This is a continuation of application Ser. No. 09/181,213 filed Oct. 27,1998, now U.S. Pat. No. 6,265,786.

TECHNICAL FIELD

This invention relates to the general field or turbogenerator controlsand more particularly to an improved high speed turbogenerator controlsystem having variable frequency output power which provides electricalpower to motors which have power requirements that normally vary in arepetitive manner over time.

BACKGROUND OF THE INVENTION

There are many industrial and commercial applications that utilizeelectrical motors to produce repetitive axial motions. The electricalmotor's rotary motion can be converted into axial motion by any numberof mechanisms such as cams, cranks, scotch yokes, or cable drums just toname a few. In any such application, the electrical power requirement ofthe motor is inherently variable and is cyclically locked to therepetitive axial motion. The motor power in these applications variesboth due to inertial effects (the need to accelerate and decelerate theaxially moving components of the system and the need to accelerate anddecelerate the rotationally moving components of the system) and due tothe work effects (changes in the work performed by the axially movingcomponents as a function of their axial position and velocity). Themagnitude of the motor power variation with time can be many times theaverage power requirement of the motor. Both the inertial effects andthe work effects can cause the motor to function as a generator whichproduces electrical power at various times in the system's cyclicalmotion.

An elevator is one well-known example of an electrical motor producingaxial motion wherein the motor's electrical power requirements vary withthe passenger load, the axial velocity of the elevator and the axialacceleration/deceleration of the elevator. Deliberate deceleration orbraking can be achieved by recovering the excess energy in theelevator's mechanical system (e.g. during the descent of a heavilyloaded elevator) utilizing regeneration to convert that mechanicalenergy into electrical energy which can go back into an electricaldistribution system.

A less well known example of a motor producing repetitive axial motionis a pump-jack type oil well. Also known as a walking beam (a large beamarranged in teeter totter fashion) or a walking-horse oil well, thepump-jack oil well generally comprises a walking beam suitably journaledand supported in an overhanging relationship to the oil well borehole sothat a string of rods (as long as two miles) can be attached to thereciprocating end of the walking beam with the other end attached to alift pump chamber at the bottom of the bore hole. A suitable drivingmeans, such as an electrical motor or internal combustion engine, isconnected to a speed reduction unit which drives a crank which in turnis interconnected to the other end of the walking beam by a pitman.

Typically, pump-jack oil wells utilize an induction motor powered byconstant frequency, three-phase electrical power from a utility grid.The pump-jack pumping cycle varies the induction motor's speed onlyslightly as allowed by plus or minus a few percent of motor slip.However, the induction motor power typically varies over the pumpingcycle by about four (4) times the average motor power level. At two (2)points in the pumping cycle, the motor power requirement peaks and attwo (2) other points, the motor power requirements are at a minimum.Typically, at one of these minimum power requirement points in thepumping cycle, the induction motor extracts enough kinetic energy and/orwork from the moving masses of the well to be able to function as agenerator and produce electrical power which must be absorbed by theutility grid.

Whether the pump-jack oil well is driven by an induction motor or by aninternal combustion engine, there is excess mechanical energy at somepoint(s) in the pumping cycle which must be absorbed to preventexcessive velocity induced stresses in the pump-jack oil well movingparts. When a pump-jack oil well is powered by an internal combustionengine, engine compression is the means by which this energy isdissipated (compression losses) while in the normal utility grid poweredinduction motor system, the induction motor is periodically driven atoverspeed causing it to return power to the utility grid.

A micro turbogenerator with a shaft mounted permanent magnetmotor/generator can be utilized to provide electrical power for a widerange of utility, commercial and industrial applications. While anindividual permanent magnet turbogenerator may only generate 24 to 50kilowatts, powerplants of up to 500 kilowatts or greater are possible bylinking numerous permanent magnet turbogenerators together. Peak loadshaving power, grid parallel power, standby power, and remote location(stand-alone) power are just some of the potential applications forwhich these lightweight, low noise, low cost, environmentally friendly,and thermally efficient units can be useful.

The conventional power control system for a turbogenerator producesconstant frequency, three-phase electrical power that closelyapproximates the electrical power produced by utility grids. If aturbogenerator with a conventional system for controlling its powergeneration were utilized to power a pump-jack type oil well, theturbogenerator's power capability would have to be sufficient to supplythe well's peak power requirements, that is, about four (4) times thewell's average power requirement. In other words, the turbogeneratorwould have to be about four (4) times as large, four (4) times as heavy,and four (4) times as expensive as a turbogenerator that only had toprovide the average power required by the oil well rather than thewell's peak power requirements.

There are other inherent difficulties present if a turbogenerator with aconventional power control system is used to provide electrical powerfor a pump-jack type of oil well. If, for example, the oil well is inthe part of the pumping cycle where it normally generates rather thanconsumes power, the operating speed of the rotating elements of theturbogenerator will tend to increase. The fuel control system of thepower control system will attempt to reduce the fuel flow to thetubogenerator combustor in order to prevent the turbogenerator'srotating elements from overspeeding which, in turn, risks quenching theflame in the combustor (flame out). A minimum fuel flow into thecombustor must be maintained to avoid flame out. This results in aminimum level of power generation, which together with the powerproduced by the oil well itself, must be deliberately dissipated aswasted power by the turbogenerator system, usually with a load resistorbut sometimes with a pneumatic load, either of which will reduce theturbogenerator system efficiency.

Also, when the power requirements for the oil well fall below the well'speak requirement, the conventional turbogenerator control system willreduce the turbogenerator speed and the turbogenerator combustiontemperature. Since the present systems do not have any means todissipate excess power, the rapidly fluctuating load levels andunloading operation produce undesirable centrifugal and thermal cyclesstresses in many components of the turbogenerator system which will tendto reduce turbogenerator life, reliability and system efficiency.

When a pump-jack type oil well is powered by constant frequencyelectrical power from a utility grid on a conventionally controlledturbogenerator, the oil extraction pumping rate may not be sufficient tokeep up with the rate at which oil seeps into the well. In this case,potential oil production and revenues may be lost. Alternately, the oilextraction pumping rate may be greater than the rate at which oil seepsinto the well. In this case, the oil well may waste power when no oil isbeing pumped or it may be necessary to shut down the oil well for aperiod of time to allow more oil to seep into the well.

For the reasons stated above, the conventional turbogenerator controlsystem is not generally suitable for pump-jack oil well systems.

SUMMARY OF THE INVENTION

The turbogenerator control system of the present invention includes ahigh frequency inverter synchronously connected to the permanent magnetmotor/generator of a turbogenerator, a low frequency load inverterconnected to the induction motor(s) of the pump-jack oil well(s), adirect current bus electrically connecting the two (2) inverters, and acentral processing unit which controls the frequency and voltage/currentof each of the inverters. This control system can readily start theturbogenerator.

Alternately, a turbogenerator control system, when utilized to generatepower, can include a bridge rectifier which converts the high frequencythree-phase electrical power produced by the permanent magnetmotor/generator of the turbogenerator into direct current power, a lowfrequency load inverter connected to the induction motor(s) of thepump-jack oil well(s), a direct current bus electrically connecting therectifier to the low frequency load inverter and a central processingunit which controls the frequency and voltage/current of the lowfrequency load inverter. The configuration of this control system can bemodified by switching electrical contactors or relays to allow the lowfrequency load inverter to be used to start the turbogenerator.

Throughout the oil well's pumping cycle, the central processing unitincreases or decreases the frequency of the low frequency load inverterin order to axially accelerate and decelerate the masses of the downhole steel pump rod(s) and oil and to rotationally accelerate anddecelerate the masses of the motor rotor and counter balance weights.

Precisely controlling the acceleration and deceleration of both theaxially moving and rotational moving masses of the oil well allowsrelatively independent control of the rate at which shaft power andelectrical power can be converted into kinetic energy. This kineticenergy can be cyclically stored by and extracted from the moving masses.Just as changing the rotational velocity versus time profile of thewell's rotating components allows the well to function as a conventionalflywheel, changing the normal axial velocity versus time profile of thewell's massive down hole moving components and oil, allows the well tofunction as an axial flywheel. Adjusting the frequency of the lowfrequency load inverter and the resulting speed of the well's inductionmotor also allows the oil pumping power to be controlled as a functionof time. The sum of the well's oil pumping power requirements and thepower converted into or extracted from the kinetic energies of themoving oil well masses is controlled so as to be nearly constant.

Thus, the combination of tailoring oil well pumping power as a functionof time and precisely controlling the insertion and extraction ofkinetic energy into and out of the moving masses of oil wells results instabilizing the power requirements demanded of a turbogenerator poweringpump-jack oil wells. This is turn allows the size of the turbogeneratorto be down sized by a factor of perhaps four to one (4 to 1), avoidsextreme variations in turbogenerator operating speed and combustiontemperature as well as avoids possible damage to the turbogeneratorcaused by cyclical variations in thermal and centrifugal stresses andpossible damage to the controller/inverter electronics caused byvariation in turbogenerator voltage.

It is, therefore, a principal aspect of the present invention to providea system to control the operation of a turbogenerator and its electronicinverters.

It is another aspect of the present invention to control the flow offuel into the turbogenerator combustor.

It is another aspect of the present invention to control the temperatureof the combustion process in the turbogenerator combustor and theresulting turbine inlet and turbine exhaust temperatures.

It is another aspect of the present invention to control the rotationalspeed of the turbogenerator rotor upon which the centrifugal compressorwheel, the turbine wheel, the motor/generator, and the bearings aremounted.

It is another aspect of the present invention to control the torqueproduced by the turbogenerator power head (turbine and compressormounted and supported by bearings on a common shaft) and delivered tothe motor/generator of the turbogenerator.

It is another aspect of the present invention to control the shaft powerproduced by the turbogenerator power head and delivered to themotor/generator of the turbogenerator.

It is another aspect of the present invention to control the electricalpower produced by the motor/generator of the turbogenerator.

It is another aspect of the present invention to control the operationsof the high frequency inverter which inserts/extracts power into/fromthe motor/generator of the turbogenerator and produces electrical powerfor the direct current bus of the turbogenerator controller.

It is another aspect of the present invention to control the operationsof the low frequency load inverter which uses power from the directcurrent bus of the turbogenerator controller to generate low frequency,three-phase power.

It is another aspect of the present invention to minimize variations inthe fuel flow rate into the turbogenerator combustor over the operatingcycle of a pump-jack oil well.

It is another aspect of the present invention to minimize variations inthe combustion and turbine temperatures of the turbogenerator over theoperating cycle of a pump-jack oil well.

It is another aspect of the present invention to minimize variations inthe operating speed of the turbogenerator over the operating cycle of apump-jack oil well.

It is another aspect of the present invention to minimize variations inthe shaft torque generated by the turbogenerator power head anddelivered to the motor/generator of the turbogenerator over theoperating cycle of a pump-jack oil well.

It is another aspect of the present invention to minimize variations inthe shaft power generated by the turbogenerator power head and deliveredto the motor/generator of the turbogenerator over the operating cycle ofa pump-jack oil well.

It is another aspect of the present invention to minimize variations inthe level of electrical power extracted from the motor/generator of theturbogenerator and converted into direct current power by the highfrequency inverter, or the bridge rectifier, over the operating cycle ofa pump-jack oil well.

It is another aspect of the present invention to minimize variations inthe level of electrical power extracted from the direct current bus andconverted into low frequency, three-phase power by the low frequencyload inverter over the operating cycle of a pump-jack oil well.

It is another aspect of the present invention to minimize variations inthe level of electrical power delivered to, and utilized by, theinduction motor(s) of the pump-jack oil well(s) over the operating cycleof a pump-jack oil well.

It is another aspect of the present invention to provide a controlsystem that sets the average frequency of the low frequency loadinverter over the operating cycle of a pump-jack oil well.

It is another aspect of the present invention to provide a controlsystem where the average frequency of the low frequency load inverterover the operating cycle of a pump-jack oil well can be set so that theoil pumping rate of the well is matched to the rate at which oil seepsinto the well from the surrounding oil ladened matrix. Thus, the wellneither runs dry nor has to produce oil at less than the well'scapacity.

It is another aspect of the present invention to provide a controlsystem that varies the instantaneous frequency of the low frequency loadinverter over the operating cycle of a pump-jack oil well.

It is another aspect of the present invention to provide a controlsystem that varies the instantaneous voltage or current of the lowfrequency load inverter over the operating cycle of a pump-jack oilwell.

It is another aspect of the present invention to provide a controlsystem where the variation in the instantaneous frequency of the lowfrequency load inverter over the operating cycle of a pump-jack oil wellis the primary means by which the system reduces the variations in powerrequired by the induction motor of the pump-jack oil well.

It is another aspect of the present invention to provide a controlsystem where the variation in the voltage or current of the lowfrequency load inverter over the operating cycle of a pump-jack oil wellis the secondary means by which the system reduces the variations inpower required by the induction motor of the pump-jack oil well andsimultaneously is the primary means by which the system controls theslip and maximizes the efficiency of the inductor motor.

It is another aspect of the present invention to provide a controlsystem with that can precisely control the insertion of kinetic energyinto, and the extraction of kinetic energy from, the moving masses ofthe pump-jack oil well over the operating cycle of the well.

It is another aspect of the present invention to provide a controlsystem that allows the rotational moving masses of the pump-jack oilwell to function as a flywheel for energy storage.

It is another aspect of the present invention to provide a controlsystem that allows the axially moving masses of the pump-jack oil wellto function as an axial flywheel for energy storage.

It is another aspect of the present invention to provide a controlsystem that can precisely control the instantaneous pumping work beingperformed by a pump-jack oil well or the instantaneous pumping workbeing extracted from a pump-jack oil well over the operating cycle ofthat well.

It is another aspect of the present invention to provide a controlsystem that causes the total of the instantaneous pumping energyrequired/produced by pump-jack oil well(s) and the instantaneous kineticenergy extracted/inserted from/into pump-jack oil well(s) to be nearlyconstant over the operating cycle of the well(s).

It is another aspect of the present invention to provide a controlsystem that utilizes the phase relationship of the pump-jack oil wellinduction motor voltage and current to both measure the resonantvelocities of the down hole rod string and to damp these resonances withappropriate modulations in the torque of the induction motor.

It is another aspect of the present invention to provide a controlsystem that soft clamps the maximum and minimum frequencies of the lowfrequency load inverter to avoid excessive rod stresses at highfrequencies, to avoid oil well pumping direction reversals, and tosimultaneously minimize the excitation of rod string resonances.

It is another aspect of the present invention to provide a controlsystem that soft clamps the maximum voltage of the low frequency loadinverter to avoid excessive voltage stresses on inverter and motorcomponents while simultaneously minimizing the excitation of rod stringresonances.

It is another aspect of the present invention to provide a controlsystem that soft clamps the D.C. bus voltage for safety.

It is another aspect of the present invention to provide a controlsystem that minimizes thermal and centrifugal stress cycle damage to theturbogenerator's combustor, recuperator, turbine wheel, compressorwheel, and other components that can be caused by variations inturbogenerator operating power level, speed or temperature and whichare, in turn, induced by the cyclical nature of pump-jack operation.

It is another aspect of the present invention to provide a controlsystem that minimizes the risk of combustor flame out that can occurwhen conventional turbogenerator fuel control systems reduce combustorfuel flow when the pump-jack's power requirements are at a minimum orare reversed during the pumping cycle.

It is another aspect of the present invention to provide a controlsystem that avoids the need for parasitic loads with their resultinginefficiencies and avoids the inefficiencies associated with off optimumoperations when fuel flow, temperature, and speed vary widely.

It is another aspect of the present invention to provide a controlsystem that allows the peak electrical power required by a pump-jack oilwell to be reduced by a factor of about four to one.

It is another aspect of the present invention to provide a controlsystem that allows the size, weight, and cost of a turbogenerator thatpowers a pump-jack oil well to be reduced by a factor of about four toone.

It is another aspect of the present invention to provide a controlsystem that allows the size, weight, and cost of the induction motorutilized by a pump-jack oil well to be reduced by a factor of about fourto one.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the present invention in general terms, referencewill now be made to the accompanying drawings in which:

FIG. 1 is a perspective view, partially cut away, of a permanent magnetturbogenerator/motor for use with the power control system of thepresent invention;

FIG. 2 is a functional block diagram of the interface between aturbogenerator/motor controller and the permanent magnetturbogenerator/motor illustrated in FIG. 1;

FIG. 3 is a functional block diagram of the permanent magnetturbogenerator/motor controller of FIG. 2;

FIG. 4 is a functional block diagram of the interface between analternate turbogenerator/motor controller and the permanent magnetturbogenerator/motor illustrated in FIG. 1;

FIG. 5 is a functional block diagram of the permanent magnetturbogenerator/motor controller of FIG. 4;

FIG. 6 a plan view of a pump-jack oil well system for use with the powercontrol system of the present invention;

FIG. 7 is a graph of power requirements in watts versus operating timein seconds for the pump-jack oil well system of FIG. 6;

FIG. 8 is a functional block diagram of the basic power control systemof the present invention; and

FIG. 9 is a detailed functional block diagram of the power controlsystem of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A permanent magnet turbogenerator/motor 10 is illustrated in FIG. 1 asan example of a turbogenerator/motor for use with the power controlsystem of the present invention. The permanent magnetturbogenerator/motor 10 generally comprises a permanent magnet generator12, a power head 13, a combustor 14 and a recuperator (or heatexchanger) 15.

The permanent magnet generator 12 includes a permanent magnet rotor orsleeve 16, having a permanent magnet disposed therein, rotatablysupported within a permanent magnet motor stator 18 by a pair of spacedjournal bearings. Radial stator cooling fins 25 are enclosed in an outercylindrical sleeve 27 to form an annular air flow passage which coolsthe stator 18 and thereby preheats the air passing through on its way tothe power head 13.

The power head 13 of the permanent magnet turbogenerator/motor 10includes compressor 30, turbine 31, and bearing rotor 36 through whichthe tie rod 29 passes. The compressor 30, having compressor impeller orwheel 32 which receives preheated air from the annular air flow passagein cylindrical sleeve 27 around the permanent magnet motor stator 18, isdriven by the turbine 31 having turbine wheel 33 which receives heatedexhaust gases from the combustor 14 supplied with air from recuperator15. The compressor wheel 32 and turbine wheel 33 are rotatably supportedby bearing shaft or rotor 36 having radially extending bearing rotorthrust disk 37. The bearing rotor 36 is rotatably supported by a singlejournal bearing within the center bearing housing while the bearingrotor thrust disk 37 at the compressor end of the bearing rotor 36 isrotatably supported by a bilateral thrust bearing. The bearing rotorthrust disk 37 is adjacent to the thrust face of the compressor end ofthe center bearing housing while a bearing thrust plate is disposed onthe opposite side of the bearing rotor thrust disk 37 relative to thecenter housing thrust face.

Intake air is drawn through the permanent magnet generator 12 by thecompressor 30 which increases the pressure of the air and forces it intothe recuperator 15. In the recuperator 15, exhaust heat from the turbine31 is used to preheat the air before it enters the combustor 14 wherethe preheated air is mixed with fuel and burned. The combustion gasesare then expanded in the turbine 31 which drives the compressor 30 andthe permanent magnet rotor 16 of the permanent magnet generator 12 whichis mounted on the same shaft as the turbine wheel 33. The expandedturbine exhaust gases are then passed through the recuperator 15 beforebeing discharged from the turbogenerator/motor 10.

The interface between the turbogenerator/motor controller 40 and thepermanent magnet turbogenerator/motor 10 is illustrated in FIG. 2. Thecontroller 40 generally comprises two bi-directional inverters, a lowfrequency load inverter 144 and a generator inverter 146. The controller40 receives electrical power 41 from a source such as a utility throughAC filter 51 or alternately from a battery through battery controlelectronics 71. The generator inverter 146 starts the turbine 31 of thepower head 13 (using the permanent magnet generator as a motor) formeither utility or battery power, and then the low frequency loadinverter 144 produces AC power using the output power from the generatorinverter 146 to draw power from the high speed permanent magnetturbogenerator 10. The controller 40 regulates fuel to the combustor 14through fuel control valve 44.

The controller 40 is illustrated in more detail in FIG. 3 and generallycomprises the insulated gate bipolar transistors (IGBT) gate drives 161,control logic 160, generator inverter 146, permanent magnet generatorfilter 180, DC bus capacitor 48, low frequency load inverter 144, ACfilter 51, output contactor 52, and control power supply 182. Thecontrol logic 160 also provides power to the fuel cutoff solenoid 62,the fuel control valve 44 and the ignitor 60. The battery controller 71connects directly to the DC bus. The control logic 160 receivestemperature signal 164, voltage signal 166, and current signal 184 whileproviding a relay drive signal 165.

Control and start power can come from either the external batterycontroller 71 for battery start applications or from the utility 41which is connected to a rectifier using inrush limiting techniques toslowly charge the internal bus capacitor 48. For grid connectionapplications, the control logic 160 commands gate drives 161 and thesolid state (IGBT) switches associated with the low frequency loadinverter 144 to provide start power to the generator inverter 146. TheIGBT switches are operated at a high frequency and modulated in a pulsewidth modulation manner to provide four quadrant inverter operationwhere the inverter 144 can either source power from the DC link to thegrid or source power from the grid to the DC link. This control may beachieved by a current regulator. Optionally, two of the switches mayserve to create an artificial neutral for stand-alone operations.

The solid state (IGBT) switches associated with the generator inverter146 are also driven from the control logic 160 and gate drives 161,providing a variable voltage, variable frequency, three-phase drive tothe generator motor 10 to start the turbine 31. The controller 40receives current feedback 184 via current sensors when the turbinegenerator has been ramped up to speed to complete the start sequence.When the turbine 31 achieves self-sustaining speed, the generatorinverter 146 changes its mode of operation to boost the generator outputvoltage and provide a regulated DC link voltage.

The generator filter 180 includes a plurality of inductors to remove thehigh frequency switching components from the permanent magnet generatorpower so as to increase operating efficiency. The AC filter 51 alsoincludes a plurality of inductors plus capacitors to remove the highfrequency switching components. The output contactor 52 disengages thelow frequency load inverter 144 in the event of a unit fault.

The fuel solenoid 62 is a positive fuel cutoff device which the controllogic 160 opens during the start sequence and maintains open until thesystem is commanded off. The fuel control valve 44 is a variable flowvalve providing a dynamic regulating range, allowing minimum fuel duringstart and maximum fuel at full load. A variety of fuel controllers,including liquid and gas fuel controllers may be utilized. The ignitor60 would normally be a spark type device, similar to a spark plug for aninternal combustion engine. It would, however, only be operated duringthe start sequence.

For stand-along operation, the turbine is started using an external DCconverter which boosts voltage from an external source such as a batteryand connects directly to the DC link. The flow frequency load inverter144 can then be configured as a constant voltage, constant frequencysource. However, the output is not limited to being a constant voltage,constant frequency source, but rather may be a variable voltage,variable frequency source. For rapid increases in output power demand,the external DC converter supplies energy temporally to the DC link andto the power output, the energy is then restored to the energy storageand discharge system 69 after a new operating point is achieved.

A functional block diagram of the interface between the alternatecontroller 40′ and the permanent magnet turbogenerator/motor 10 forstand-alone operation is illustrated in FIG. 4. The generator controller40′ receives power from a source such as a utility or battery system tooperate the permanent magnet generator 12 as a motor to start rotationof compressor 30 and turbine 31 of the power head 13. During the startsequence, the utility power 41 if available, is rectified and acontrolled frequency ramp is supplied to the permanent magnet generator12 which accelerates the permanent magnet rotor 16, the compressor wheel32, bearing rotor 36 and turbine wheel 33. The acceleration provides anair cushion for the air bearings and airflow for the combustion process.At about 12,000 rpm, spark and fuel are provided to the combustor 14 andthe generator controller 40′ assists acceleration of the turbogenerator10 up to about 40,000 rpm to complete the start sequence. The fuelcontrol valve 44 is also regulated by the generator controller 40′.

Once self sustained operation is achieved, the generator controller 40′is reconfigured to produce low frequency, variable voltage three-phaseAC power (up to 250 VAC for 208 V systems, up to 550 VAC for 480 Vsystems) 42 from the rectified high frequency AC output (280-380 voltsfor 280 V systems, 600-900 volts for 480 V systems) of the high speedpermanent magnet turbogenerator 10 to supply the needs of the pump-jackoil well induction motor. The permanent magnet turbogenerator 10 iscommanded to a power set point with fuel flow, speed, and combustiontemperature varying as a function of the desired output power.

The generator controller 40′ also includes an energy storage anddischarge system 69 having an ancillary electric storage device 70 whichis connected through control electronics 71. This connection isbi-directional in that electrical energy can flow from the ancillaryelectric storage device 70 to the generator controller 40′, for exampleduring turbogenerator/motor start-up, and electrical energy can also besupplied from the turbogenerator/motor controller 40′ to the ancillaryelectric storage device or battery 70 during sustained operation.

An example of this alternate turbogenerator/motor control system isdescribed in U.S. patent application No. 003,078, filed Jan. 5, 1998 byEverett R. Geis, Brian W. Peticolas, and Joel B. Wacknov entitled“Turbogenerator/Motor Controller with Ancillary EnergyStorage/Discharge”, assigned to the same assignee as this applicationand incorporated herein by reference.

The functional blocks internal to the generator controller 40′ areillustrated in FIG. 5. The generator controller 40′ includes in seriesthe start power contactor 46, bridge rectifier 47, DC bus capacitors 48,pulse width modulated (PWM) inverter 49, AC output filter 51, outputcontactor 52, generator contactor 53, and permanent magnet generator 12.The generator rectifier 54 is connected from between the bridgerectifier 47 and bus capacitors 48 to between the generator contactor 53and permanent magnet generator 12. The AC power output 42 is taken fromthe output contactor 52 while the neutral is taken from the AC filter51.

The control logic section consists of control power supply 56, controllogic 57, and solid state switched gate drives illustrated as integratedgate bipolar transistor (IGBT) gate drives 58, but may be drives for anyhigh speed solid state switching device. The control logic 57 receives atemperature signal 64 and a current signal 65 while the IGBT gate drives58 receive a voltage signal 66. The control logic 57 sends controlsignals to the fuel cutoff solenoid 62, the fuel control valve(s) 44(which may be a number of electrically controlled valves), the ignitor60 and compressor discharge air dump valve 61. AC power 41 is providedto both the start power contactor 46 and in some instances directly tothe control power supply 56 in the control logic section of thegenerator controller 40′ as shown in dashed lines.

The energy storage and discharge system 69 is connected to thecontroller 40′ across the voltage bus V_(bus) between the bridgerectifier 47 and DC bus capacitor 48 together with the generatorrectifier 54. The energy storage and discharge system 69 includes anoff-load device 73 and ancillary energy storage and discharge switchingdevices 77 both connected across voltage bus V_(bus).

The off-load device 73 includes an off-load resistor 74 and an off-loadswitching device 75 in series across the voltage bus V_(bus). Theancillary energy storage and discharge switching device 77 comprises acharge switching device 78 and a discharge switching device 79, also inseries across the voltage bus V_(bus). Each of the charge and dischargeswitching devices 78, 79 include solid state switches 81, shown as anintegrated gate bipolar transistor (IGBT) and anti-parallel diodes 82.Capacitor 84 and ancillary storage and discharge device 70, illustratedas a battery, are connected across the discharge switching device 79with main power relay 85 between the capacitor 84 and the ancillaryenergy storage and discharge device 70. Inductor 83 is disposed betweenthe charge switching device 78 and the capacitor 84. A precharge device87, consisting of a precharge relay 88 and precharge resistor 89, isconnected across the main power relay 85.

The PWM inverter 49 operates in two basic modes: a variable voltage(0-190 V line to line), variable frequency (0-700 Hertz) constant voltsper Hertz, three-phase mode to drive the permanent magnetgenerator/motor 12 for start up or cool down when the generatorcontactor 53 is closed; or a constant voltage (120 V line to neutral perphase), constant frequency three-phase 60 Hertz mode. The control logic57 and IGBT gate drives 58 receive feedback via current signal 65 andvoltage signal 66, respectively, as the turbine generator is ramped upin speed to complete the start sequence. The PWM inverter 49 is thenreconfigured to provide 60 Hertz power, either as a current source forgrid connect, or as a voltage source.

The PWM inverter 49 is truly a dual function inverter which is used bothto start the permanent magnet turbogenerator/motor 10 and to convert thepermanent magnet turbogenerator/motor output to utility power, either assixty Hertz, three-phase, constant voltage for stand alone applications,or as a sixty Hertz, three-phase, current source for grid parallelapplications. With start power contactor 46 closed, single orthree-phase utility power is brought to bridge rectifier 47 and provideprecharged power and then start voltage to the bus capacitors 48associated with the PWM inverter 49. This allows the PWM inverter 49 tofunction as a conventional adjustable speed drivel motor starter to rampthe permanent magnet turbogenerator/motor 10 up to a speed sufficient tostart the gas turbine 31.

An additional rectifier 54, which operates from the output of thepermanent magnet turbogenerator/motor 10, accepts the three-phase power,(up to 380 volt AC) from the permanent magnet generator/motor 12 (whichat full speed produces 1600 Hertz power). This diode is classified as afast recovery diode rectifier bridge. Six diode elements arranged in aclassic bridge configuration comprise this high frequency rectifier 54which provides output power DC to power the inverter. Alternately, therectifier 54 may be replaced with a high speed inverter permanentlyconnected to the turbogenerator, eliminating the dual functionality ofthe inverter 49, and eliminating the need for certain contactors, suchas generator contactor 53. The rectified voltage is as high as 550 voltsunder no load.

The off-load device 73, including off-load resistor 74 and off-loadswitching device 75 can absorb thermal energy from the turbogenerator 10when the load terminals are disconnected, or there is a rapid reductionin load power demand. The off-load switching device 75 will turn onproportionally to the amount of off-load required and essentially willprovide a load for the gas turbine 31 while the fuel is being cut backto stabilize operation at a reduce power level. The system serves as adynamic brake with the resistor connected across the DC bus through anIGBT and serves as a load on the gas turbine during any overspeedcondition.

In addition, the ancillary electric storage device 70 can continuemotoring the turbogenerator 10 for a short time after a shutdown inorder to cool down the turbogenerator 10 and prevent the soak back ofheat from the recuperator 15. By continuing the rotation of theturbogenerator 10 for several minutes after shutdown, the power head 13will keep moving air through the turbogenerator which will sweep heataway from the permanent magnet generator 12 and compressor wheel 32.This allows a gradual and controlled cool down of all of the turbine endcomponents.

The battery switching devices 77 are a dual path since the ancillaryelectric storage device 70 is bi-directional. The ancillary electricstorage device 70 can provide energy to the power inverter 49 when asudden demand or load is required and the gas turbine 31 is not up tospeed. At this point, the battery discharge switching device 79 turns onfor a brief instant and draws current through the inductor 83. Thebattery discharge switching device 79 is then opened and the currentpath continues by flowing through the diode 82 of the battery chargeswitching device 78 and then in turn provides current into the invertercapacitor 48.

The battery discharge switching device 79 is operated at a varying dutycycle, high frequency, rate to control the amount of power and can alsobe used to initially ramp up the controller 40′ voltage during batterystart operations. After the system is in a stabilized self-sustainingcondition, the battery charge switching device 78 is used in an oppositemanner. At this time, the battery charge switching device 78periodically closes in a high frequency modulated fashion to forcecurrent through inductor 83 and into capacitor 84 and then directly intothe ancillary electric storage device 70.

The capacitor 84, connected to the ancillary electric storage device 70via the precharge relay 88 and resistor 89 and the main power relay 85,is provided to buffer the ancillary electric storage device 70. Thenormal, operating sequence is that the precharge relay 88 is momentarilyclosed to a low charging of all of the capacitive devices in the entiresystem and then the main power relay 85 is closed to directly connectthe ancillary electric storage device 70 with the control electronics71. While the main power relay 85 is illustrated as a switch, it mayalso be a solid state switching device.

FIG. 6 generally illustrates a pump-jack oil well system with a pumpingunit 110 having a driving means 111 connected thereto with the apparatussuitably supported on base 112. A Samson post 113 supports a walkingbeam 114 which is pivotably affixed thereto by a saddle 115 which formsa journal.

The walking beam 114 has a horse-head attachment 116 at one end thereofso that a cable 117 can be connected at yoke 118 (including a load cellto provide real time monitoring of the rod load and its dynamic behaviorincluding its resonant frequencies and resonant motions) to a polishedrod 119 to enable a rod string located downhole in the well bore 120 tobe reciprocated. The other end 121 of the walking beam 114 is journaledat 122 to a pitman or connecting rod 123. The other end of theconnecting rod 123 is affixed to a crank 124 by mean of journal 125. Thecrank 124 is affixed to a power output drive shaft 126 of a reductiongear assembly 127 with a counterbalance 128 affixed along a marginallyfree end portion of the crank 124.

The gear reducer 127 is mounted on a support 129 which is in turnmounted on the base 112. Driven gear or pulley 130 is attached by meansof belts or chains 131 to the drive gear or pulley 132 which in turn issupported at 133 from base 134. An electrical induction motor 137 isadjustably mounted by hinge means on the support 133. The electricalinduction motor 137 may include a rotating inertial mass 138.

FIG. 7 illustrates a graph of power requirements in watts versusoperating time in seconds for the pump-jack oil well system generallydescribed in FIG. 6 with power supplied from a utility grid. Region “A”represents the start of the pump-jack stroke. The crank arm 124 andcounterweight 128 of the pump-jack passes through top dead center andthe sucker rod begins its upward travel at approximately top deadcenter, depending on the exact positioning of the crankshaft center 126with respect to the beam journal 122, and may be several degrees eitherside of top dead center. The induction motor power flows to thepump-jack until the crank arm is approximately thirty (30) degrees aftertop dead center at which point energy from the falling counterweightbegins to contribute significantly to the liquid load pumping power(displacing motor power)

In region “B”, energy released by the falling counterweight on the crankarm exceeds the liquid pumping load and tries to overspeed the drivemotor turning it into a generator. During this period, electrical poweris exported to the utility grid. In region “C”, the counterweight haspassed through bottom dead center and is rising. The sucker rod istravelling down under its own weight and the motor power goes almostexclusively to lifting the counterweight. Region “D” represents theperiod of time in the cycle when the counterweight is being raised andthe sucker rod lowered while the liquid lift load occurs during Region“E”.

More specifically, bottom dead center on the crank arm occurs atapproximately five (5) seconds on the above scale. Between five andon-half (5½) seconds and eight and one-half (8½) seconds, thecounterweight is being raised as the sucker rod lowers. The peakelectrical demand of approximately twenty-six (26) kWe occurs nearlyninety (90) degrees after bottom dead center. At eight and one-half (8½)seconds, the counterweight crosses top dead center where the liquid loadis imposed.

At this point, there is little energy available from the counterweightas it is moving essentially horizontal so a secondary power peak occursas liquid is being lifted before the counterweight begins to fall. Ateleven (11) seconds, the falling counterweight delivers more power(torque) than required for liquid lift and the motor overspeeds(slightly) turning the motor into a generator that brakes thecounterweight. Peak power generated is approximately eight (8) kW. Aboutthirty (30) degrees after bottom dead center, the crank slows to belowsynchronous speed for the motor at which point power is required to liftthe counterweight again.

The basic power system of the present invention is illustrated in blockdiagram form in FIG. 8. The power control system includes theturbogenerator controller 40 or 40′, the turbogenerator 10, thepump-jack induction motor 137, and the pump-jack oil well 110. Thecontroller 40 or 40′ regulates the turbogenerator speed required toproduce the power required by the pump-jack by varying the fuel flow tothe turbogenerator combustor 14 while the controller 40 or 40′specifically varies the output frequency of inverter 144 or 49 and thespeed of the pump-jack induction motor 137 to control the load power andto maintain turbogenerator operation within overspeed, combustor flameout and overtemperature limits.

FIG. 9 illustrates a more detailed functional block diagram of the powercontrol system of the present invention which includes three primarycontrol loops used to regulate the turbogenerator gas turbine engine.The three primary control loops are the turbine exhaust gas temperaturecontrol loop 200, the turbogenerator speed control loop 202, and thepower control loop 204. The speed control loop 202 commands fuel outputto the turbogenerator fuel control 44 to regulate the rotating speed ofthe turbogenerator 10. The turbine exhaust gas temperature control loop200 commands fuel output to the fuel control 44 to regulate theoperating temperature of the turbogenerator 10. The minimum fuel command210 is selected by selector 212 which selects the least signal from thespeed control loop 202 and the turbine exhaust gas temperature controlloop 200.

The pump-jack load profile, as illustrated in FIG. 7, consists ofperiods of variable load and periods or regenerative power generation(region B of FIG. 7). The possibility of turbogenerator overspeed canresult, particularly when a stored thermal energy device such as arecuperator 15 is utilized as part of the turbogenerator 10. To preventthis overspeed and maximize the overall system efficiency, the pump-jackspeed can be increased to provide an inertial load and an increased oilpumping load which counter the regenerative load.

This is accomplished in part by a maximum turbogenerator speed controlloop 214 that varies the frequency command to the low frequency loadinverter 144 or to the variable speed inverter 49, which varies thespeed of the induction motor 137 of the pump-jack 110. The frequencyoffset signal 279 is produced from limitor 287. In addition, the speedof the pump-jack induction motor 137 can be varied to control maximum ortransient turbine exhaust gas temperature by a maximum turbine exhaustgas temperature control loop 216. The frequency offset signal 218 isproduced from limitor 280.

The turbogenerator power control system of the present invention and theturbogenerator 10 which it controls are capable of being utilized bypump-jack oil well operators without the need for any special training.The turbogenerator 10 and control system are also capable of being movedfrom one group of one or more oil wells to another group of wellswithout any requirement to manually change any of the control systemparameters.

The power control system can automatically adapt itself to powering anynumber of wells from one to the maximum number of oil wells permitted bythe power level available from the turbogenerator 10 and can tolerateall of the oil wells requiring peak power at the same time or havingpeak power requirements staggered in time (out of phase). It cantolerate the total power required by the oil wells that it suppliesbeing near the peak power capability of the turbogenerator 10 or beingzero (e.g. with open circuit breakers), or anywhere in between.

As illustrated in FIG. 9, the average frequency 240 that is desired forthe three-phase electrical power produced by the low frequency or loadinverter 144 (or 49) is compared in summer or comparator 242 with theinstantaneous frequency 243 produced by the inverter 144 (or 49). Thedifference in these frequency values, the error signal 244, is utilizedas the input to a turbogenerator speed command control loop 230 and aturbogenerator power command control loop 232. When the average overtime of the error signal 244 is zero, the power utilized by the oilwells is equal to the power generated by the turbogenerator 10.

The turbogenerator speed command control loop 230, includingproportional integral control 231, generates a recommended speed signal245 for the turbogenerator 10 that should produce a level of electricalpower equal to the power utilized by the oil wells. This recommendedspeed signal 245 is limited by limitor 246 to a maximum value equal tothe maximum safe operating speed of the turbogenerator 10 and also islimited by the limitor 246 to a minimum value equal to the minimum speedat which the turbogenerator 10 can operate with no power output.

The proportional integral control 233 of the power command control loop232 establishes a recommended power consumption level signal 234 for theoil wells that should match the level of electrical power produced bythe turbogenerator 10. This recommended power consumption level signal234 is limited by limitor 236 to a maximum value equal to the maximumpower that can be produced by the turbogenerator 10 and is furtherlimited by limitor 236 to a minimum value equal to zero when the oilwell's circuit breakers are open.

The output signal 247 from the speed command control loop 230constitutes a speed command 247 to the turbogenerator 10. This speedcommand 247 is compared in comparator 248 against the realturbogenerator speed feedback signal 206 from the turbogenerator 10. Theerror signal 249 between these two speed values is fed to theproportional integral control 203 of the speed control loop 202 toproduce a recommended fuel flow signal 258.

The look up table 208 is used together with the real turbogeneratorspeed feedback signal 206 from the turbogenerator 10 to establish therecommended turbine exhaust gas temperature command 250 for the turbine.This recommended turbine exhaust gas temperature command 250 is comparedin comparator 251 against the real turbine exhaust gas temperaturefeedback signal 207 from the turbogenerator 10 to produce a computedturbine exhaust gas temperature error signal 252. This computed turbineexhaust gas temperature error signal 252 in inputted into proportionalintegral control 254 in the turbine exhaust gas temperature loop 200which computes a recommended fuel flow signal 256 that should eliminatethe temperature error.

Selector 212 selects the lowest of the signals from the turbine exhaustgas temperature loop 200 and the speed control loop 202 and provides thelower signal to the limitor 260 which limits the recommended fuel flowto a maximum value equal to that required to produce the maximum powerthat the turbogenerator 10 produces and to a minimum value equal to thefuel flow below which the combustor 14 will experience flame out. Theselected fuel flow value 262 is then used by the fuel control 44 todetermine/deliver the required fuel flow rate to the combustor 14. Theresulting turbogenerator speed feedback signal 206 and turbine exhaustgas temperature feedback signal 207 are measured at the turbogenerator10 and utilized elsewhere in the power control system.

The output 237 from limitor 236 constitutes the low frequency loadinverter 144 (or 49) average power command which is compared incomparator 264 with the real instantaneous power feedback signal 265from the power sensor 270. The resulting error signal 266 is utilized inproportional integral control 268 to produce a recommended instantaneousinverter frequency signal 269 that should eliminate the power error.

Comparator 271 compares the speed feedback signal 206 from theturbogenerator 10 with the maximum safe speed signal 272 for theturbogenerator 10 to produce a speed error signal 273. If the speed ofthe turbogenerator 10 is greater than the maximum safe speed 272, theproportional integral control 274 establishes a recommended frequencyincrease signal 279 (limited in limitor 287) in the low frequency loadinverter frequency and hence the pump-jack oil well speed that shouldeliminate the turbogenerator overspeed.

The turbine exhaust gas temperature feedback signal 207 from theturbogenerator 10 is compared with the maximum safe turbine exhaust gastemperature signal 275 in comparator 276 to produce an error signal 277.If the turbine exhaust gas temperature of the turbogenerator 10 isgreater than the maximum safe temperature 275, the proportional integralcontrol 278 establishes a recommended frequency increase signal 218(limited in limitor 280) in the low frequency load inverter frequencyand hence the pump-jack oil well speed that should eliminate the overtemperature.

Both of the two inverter frequency reduction signals 279 and 218 areprovided to comparator or summer 282 which also receives signal 269. Theerror signal 291 from summer 282 is provided to limitor 289 before goingto the inverter 144 or inverter 49. This limited error signal controlsthe frequency of the inverter 144 and provides a frequency limit signal243 to both comparator 242 and to the look up table 290 which computesthe inverter output voltage.

The turbogenerator 10 and pump-jack oil wells 110 are deliberatelyoperated at nearly constant power over the oil well's pumping cycle.Since, however, induction motors nominally have a power capability thatis proportional to the motor's speed and the inductive impedance and theelectromotive force generated voltage of the induction motor forconstant current are both nominally proportional to inverter frequencyand motor speed, operating the induction motor at constant voltage asthe inverter/motor frequency varies can produce unacceptable results.Such operation can, for instance, cause the motor laminations tomagnetically saturate at low frequency/speed, resulting in excessivecurrent/heating and stator winding damage. Varying induction motorvoltage approximately with the square root of inverter frequency is aviable alternative and allows the induction motor slip to be a lowexponential (e.g. 0.5) inverse function of frequency/speed (the lowerthe frequency/speed the greater the slip).

The three-phase electrical power produced by the low frequency loadinverter 144 passes through the power sensor 270. The signal 265 fromthe power sensor 270 is utilized by comparator 264 to assure that thepower delivered by the low frequency inverter 144 to the pump-jackinduction motor 137 is equal to the turbogenerator/motor power that isrequired to maintain the low frequency load inverter's average frequencyat the desired level.

The desired average frequency of the low frequency load inverter 144 canbe set equal to utility frequency (e.g. 50 or 60 Hertz) or it can be setto assure that the oil well pumps oil at the same rate as the oil seepsinto the well from the surrounding strata.

Relatively independent control of the rate at which shaft power andelectrical power can be converted into kinetic energy can be achieved byprecisely controlling the acceleration and deceleration of both theaxially moving and rotationally moving masses of the oil well. Thiskinetic energy can be cyclically stored by and extracted from the movingmasses. In other words, changing the normal axial velocity versus timeprofile of the well's massive down hole moving components and oil allowsthe well to function as what can best be described as an “axialflywheel”. Adjusting the frequency of the low frequency load inverterand the resulting speed of the well's induction motor also allows theoil pumping power to be controlled as a function of time. The sum of thewell's oil pumping power requirements and the power converted into andextracted from the kinetic energies of the moving oil well masses iscontrolled so as to be nearly constant. Without this control system thepower requirements of this type of oil well can vary over severalseconds (typically eight (8)) by up to four (4) times the average powerrequired by the well. This means that the size of the turbogeneratormight otherwise have to be increased by a factor of four (4) and theturbogenerator might otherwise experience cyclical variations inoperating speed and temperature, suffer excessive centrifugal andthermal stresses, and operate unstably and operate with low efficiency.

The improved power control system for the turbogenerator will allow aturbogenerator to provide electrical power to one or more periodicallyvarying loads, such as the induction motors of pump-jack type oil wells,without the need to vary turbogenerator operating speed, fuelconsumption or combustion temperature.

The required induction motor speed variances can be decreased byincreasing induction motor inertia, for example, by the use of theinertial mass 138. Varying pump speed, augmenting inertia energystorage, and/or using an electrical energy storage device can all beused individually or in any combination to resolve energy regenerationand/or flatten the induction motor load profile.

While specific embodiments of the invention have been illustrated anddescribed, it is to be understood that these are provided by way ofexample only and that the invention is not to be construed as beinglimited thereto but only by the proper scope of the following claims.

What we claim is:
 1. A system, comprising: a turbogenerator; a cyclicmotion machine driven by an electric motor; a low frequency inverterpowered by said turbogenerator, said inverter connected to said electricmotor; and a controller controlling said turbogenerator and saidinverter to establish a variable frequency time profile for each cycleof said cyclic motion machine and to vary the frequency of said inverteraccording to said variable frequency time profile to provide a generallyconstant power level to said electric motor.